Participation of Nitrate Sensor NRT1.1 in the Control of Cytokinin Level and Root Elongation under Normal Conditions and Nitrogen Deficit

NRT1.1 nitrate transporter acts as a nitrate sensor in some plant responses. We tried to check if it may be involved in the control of cytokinin level in the plants known to be involved in the growth responses to nitrate level. The experimental objects were Arabidopsis thaliana plants of the original ecotype Columbia (Col-0) and chl1-5 mutants. The effects of the NRT1.1 gene mutation in chl1-5 plants on hormonal and growth responses to nitrogen starvation were studied. Two types of growing conditions were used: (1) plants were placed on either standard Hoagland–Arnon or modified solution, where potassium and calcium nitrates were substituted with their chlorides; (2) plants were placed on Pryanishnikov medium, where ammonium nitrate serves as the source of nitrogen and nitrogen deficiency being modeled by its withdrawal from the medium. It has been first shown that mutation of the NRT1.1 resulted in a decline in cytokinin level in the roots of chl1-5 mutants, while roots of wild type plants were longer in accordance with lower cytokinin content in them; this hormone is known to inhibit root elongation. Cytokinin content decreased in A. thaliana, Columbia ecotype, paralleled by acceleration of root elongation in response to both variants of nitrogen starvation, while chl1-5 roots responded in this way only when nitrogen was withdrawn from Pryanishnikov solution. Substitution of nitrates by chlorides in the Hoagland–Arnon solution had no effects on either chl1-5 roots’ length or cytokinin content in them. The results suggested the involvement of NRT1.1 transceptor in the control of cytokinin level and root elongation rate in the nitrate but not in ammonium starved plants, confirming the specificity of response.